1. Field of the Invention
The invention disclosed herein relates to a sensor for measuring at least one of temperature and pressure. In particular, the sensor is used with a logging instrument in a borehole.
2. Description of the Related Art
In exploration for hydrocarbons, it is important to make accurate measurements of various properties of geologic formations. In particular, it is important to determine the various properties with a high degree of accuracy so that drilling resources are used efficiently.
Generally, oil and gas are accessed by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. Well logging can also be used to take measurements of conditions in the boreholes. The conditions in the boreholes are important to know to safely and efficiently use drilling resources.
In one embodiment, a “logging instrument” is lowered on the end of a wireline into a borehole. The logging instrument sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” Many types of measurements are made to obtain information about the geologic formations and conditions in the borehole. Two important logs are a temperature log and a pressure log.
The temperature log records temperature in the borehole at various depths. The temperature log can provide indication of temperature gradients in the borehole. The temperature log can be compared to a reference temperature log. Departures from the reference temperature log can indicate entry of fluids into the borehole. Conversely, the departures can indicate fluids exiting the borehole. In addition, the temperature log can be used to detect leaks in a borehole casing or leaks from a valve.
The pressure log records pressure at various depths within the borehole. Accurate pressure measurements can be used to monitor depletion of reservoirs associated with the production of hydrocarbons. Further, accurate measurements of pressure in the borehole are needed during drilling operations. It is important to monitor pressure during drilling operations to keep the pressure under control. If the pressure is not kept under control, then an uncontrolled release of oil and gas to the surface (known as a “blowout”) can result. The blowout can cause personal injuries, drilling rig damage, environmental damage, and damage to underground reservoirs.
Another important measurement for analyzing a formation is a chemical analysis of materials that may be found in the borehole or extracted from the formation. Filtrates are examples of fluids that may be extracted from the formation. Chemical analysis can provide information to a petro-analyst trying to determine the composition of the formation and the potential for the formation to contain a reservoir of hydrocarbons.
Therefore, what are needed are techniques to measure temperature, pressure and a chemical property within a borehole. In particular, the techniques provide for high accuracy measurements.